In test configurations, a jet engine exhaust is sometimes discharged into a duct or pipe to carry it out of, and away from, the test chamber, e.g., in large-scale jet engine tests with hot flows. In addition to the “regular” jet exhaust noise, unwanted high intensity noise is sometimes encountered in such test facilities. The unwanted noise is primarily due to the duct resonance modes excited by the jet exhaust. When the preferred mode frequency of the jet matches a duct resonant frequency there can be a locked-in super resonance or howl. Even in the absence of the locked-in resonance noise, high levels of unwanted noise may occur due to the duct modes excited by broadband disturbances of the jet. The latter noise is referred to in the following as ‘excited duct mode noise’ while the intense noise due to super resonance is referred to in the following as ‘howl’. The howl is a special case of the excited duct mode noise, and the terminology unwanted noise is used to cover both the howl and the excited duct mode noise.
The unwanted noise is problematic and can obstruct jet engine test efforts hindering aeroacoustic measurements and interfering with flow data. It can be difficult or impossible to obtain accurate test measurements of aspects of the engine's performance in the presence of the unwanted noise. Worst cases of howl may involve increased unsteady aerodynamic loads raising structural concerns of damage to the engine and/or the test setup.
Various methods for suppression of the howl have been explored, however, attempts to solve this problem have not been particularly successful. For example, protrusions or tabs placed at the periphery of the inlet of the duct and longitudinal fins located inside the duct have been found to be ineffective. A rod inserted perpendicular to the exhaust flow at different axial locations may be effective for suppressing the howl but must be painstakingly positioned, maneuvered or adjusted for a specific test setup through trial and error. Furthermore, even if a rod is effective for suppressing the howl, tests show that neither the rod nor the tabs are effective for suppressing the excited duct mode noise.